Introduce GIT_ASSERT macros

Provide macros to replace usages of `assert`.  A true `assert` is
punishing as a library.  Instead we should do our best to not crash.

GIT_ASSERT_ARG(x) will now assert that the given argument complies to
some format and sets an error message and returns `-1` if it does not.

GIT_ASSERT(x) is for internal usage, and available as an internal
consistency check.  It will set an error message and return `-1` in the
event of failure.

include/git2/errors.h

@@ -107,7 +107,8 @@ typedef enum {
 	GIT_ERROR_PATCH,
 	GIT_ERROR_WORKTREE,
 	GIT_ERROR_SHA1,
-	GIT_ERROR_HTTP
+	GIT_ERROR_HTTP,
+	GIT_ERROR_INTERNAL
 } git_error_t;
 
 /**

src/common.h

@@ -95,6 +95,33 @@
 #define NETIO_BUFSIZE DEFAULT_BUFSIZE
 
 /**
+ * Assert that a consumer-provided argument is valid, setting an
+ * actionable error message and returning -1 if it is not.
+ *
+ * Note that memory leaks can occur in a release-mode assertion
+ * failure -- it is impractical to provide safe clean up routines in these very
+ * extreme failures, but care should be taken to not leak very large objects.
+ */
+#define GIT_ASSERT_ARG(expr) do { \
+		if (!(expr)) { \
+			git_error_set(GIT_ERROR_INVALID, \
+				"invalid argument: '%s'", \
+				#expr); \
+			return -1; \
+		} \
+	} while(0)
+
+/** Internal consistency check to stop the function. */
+#define GIT_ASSERT(expr) do { \
+		if (!(expr)) { \
+			git_error_set(GIT_ERROR_INTERNAL, \
+				"unrecoverable internal error: '%s'", \
+				#expr); \
+			return -1; \
+		} \
+	} while(0)
+
+/**
  * Check a pointer allocation result, returning -1 if it failed.
  */
 #define GIT_ERROR_CHECK_ALLOC(ptr) if (ptr == NULL) { return -1; }

tests/core/assert.c

+#include "clar_libgit2.h"
+
+static const char *hello_world = "hello, world";
+
+static int dummy_fn(const char *myarg)
+{
+	GIT_ASSERT_ARG(myarg);
+	GIT_ASSERT_ARG(myarg != hello_world);
+	return 0;
+}
+
+static int bad_math(void)
+{
+	GIT_ASSERT(1 + 1 == 3);
+	return 42;
+}
+
+void test_core_assert__argument(void)
+{
+	cl_git_fail(dummy_fn(NULL));
+	cl_assert(git_error_last());
+	cl_assert_equal_i(GIT_ERROR_INVALID, git_error_last()->klass);
+	cl_assert_equal_s("invalid argument: 'myarg'", git_error_last()->message);
+
+	cl_git_fail(dummy_fn(hello_world));
+	cl_assert(git_error_last());
+	cl_assert_equal_i(GIT_ERROR_INVALID, git_error_last()->klass);
+	cl_assert_equal_s("invalid argument: 'myarg != hello_world'", git_error_last()->message);
+
+	cl_git_pass(dummy_fn("foo"));
+}
+
+void test_core_assert__internal(void)
+{
+	cl_git_fail(bad_math());
+	cl_assert(git_error_last());
+	cl_assert_equal_i(GIT_ERROR_INTERNAL, git_error_last()->klass);
+	cl_assert_equal_s("unrecoverable internal error: '1 + 1 == 3'", git_error_last()->message);
+}